The default was to compile every logs (including debug) and set
the default level to debug.
As some debug logs may hurt performance, a notice is added and the
default level is now info.
In order to enable debug logs, they must be compiled with
RTE_LOG_LEVEL=RTE_LOG_DEBUG and enabled at runtime with --log-level=8.
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>